The present invention relates generally to data storage systems and, more particularly, to a system and method for identifying and filtering a frequency associated with undesirable resonance in a mechanical support assembly for a read/write head.
It is well understood in the disk drive system manufacturing community that mechanical head/suspension/arm (HSA) assembly resonances create problems of varying severity during proper servo tracking operations in a disk drive system. These undesirable mechanical resonances are typically identified during manufacturing and screening by performing frequency analysis of the position error signal (PES) of each individual read/write head. The variations in similar suspension resonance frequencies between different HSA assemblies can be substantial. For example, it is not uncommon to observe more than a +/xe2x88x92200 Hz frequency variation in bending and torsional modes between suspensions in a single disk drive system. It is also not uncommon for a disk drive system to have upwards to 20 or more HSA assemblies.
A significant difficulty associated with addressing unwanted mechanical HSA assembly resonances within the disk drive servo loop is the present inability to precisely identify undesirable resonance frequencies derived from PES measurements. One reason for this present inability is the limited servo sampling rate associated with typical disk drive systems. For example, a given disk drive system may employ 90 servo sectors for each of a number of disks spinning at 7,200 revolutions per minute (RPM). Such a disk drive system will have a servo sampling rate of. 90xc2x77,200/60=10.8 kHz, which provides for a low frequency resolution of 7,200/60=120 Hz. This relatively low frequency resolution of 120 Hz implies that an HSA assembly resonance can only be identified within an accuracy of +/xe2x88x9260 Hz. It will be appreciated that future disk drive systems having disks spinning at 10,000 RPM or higher will demonstrate even poorer frequency resolution. For example, a disk drive system employing a data storage disk having 80 servo sectors per revolution and spinning at 10,000 RPM will have a frequency resolution of only 166.7 Hz.
There exists a need for a system and method for identifying frequencies of bothersome mechanical HSA assembly resonances occurring in a data storage system with a high degree of precision. There exists a further need for a system and method for adaptively filtering such bothersome mechanical HSA assembly resonances with a high degree of effectiveness. The present invention fulfills these and other needs.
The present invention is directed to a method and apparatus for identifying and filtering a resonance frequency of a support structure supporting a read/write head in proximity with a data storage medium. A position error signal is produced using servo information provided on a data storage medium, and a particular sample of a frequency transform of the position error signal is obtained. A magnitude of the particular sample is computed, preferably using Goertzel""s algorithm. The velocity of the data storage medium is varied, and the magnitude of a particular PES frequency transform sample is computed at each of the velocities. A resonance frequency of the support structure is calculated using the velocity associated with the particular sample having a maximum magnitude.
The frequency transform of the position error signal may be produced using one of several frequency transform techniques, including a Discrete Fourier Transform (DFT), Fast Fourier Transform (FFT), a Short-Time Discrete Fourier Transform (STFT), or a wavelet transform technique. Obtaining the particular sample of a PES frequency transform in accordance with one embodiment involves obtaining a PES frequency transform sample which is representative of a frequency higher than the resonance frequency of the support structure. The velocity of the data storage medium is reduced during the resonance frequency identification process according to this embodiment.
In accordance with another embodiment, obtaining the particular sample involves obtaining a PES frequency transform sample which is representative of a frequency lower than the resonance frequency of the support structure. The velocity of the data storage medium is increased during the resonance frequency identification process according to this embodiment.
Another aspect of the present invention concerns canceling the effect of one or more resonance frequencies associated with a read/write head suspension in a position error signal. In accordance with one embodiment, one or more transducers supported on respective support structures are situated in proximity with the data storage medium. Each of the transducers are defined within a respective servo loop. A position error signal is produced within each of the servo loops using a respective one of the transducers. One or more programmable filters are provided within each of the servo loops. A processor programs selected ones of the programmable filters to substantially cancel an effect of one or more resonance frequencies associated with each of the support structures in the respective position error signals.
In a further aspect of the present invention, each of the selected programmable filters adds lag into its respective servo loop, and the processor adjusts to compensate for this additional lag. The processor may, for example, compensate for the additional lag by adding lead into the respective servo loops. The processor may comprise a Proportional-Integral-Differential (PID) controller, and the programmable filters may comprise programmable notch filters or programmable elliptic filters.
A method and apparatus for identifying a resonance frequency of a read/write head support structure according to the principles of the present invention may be implemented in a data storage system and, preferably, implemented in-situ a data storage system without resort to circuitry external to the data storage system.
The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. Advantages and attainments, together with a more complete understanding of the invention, will become apparent and appreciated by referring to the following detailed description and claims taken in conjunction with the accompanying drawings.